For production of a polyimide film, a film having self-supporting property, which is made from a polyimide precursor such as a polyamic acid solution and the like (hereinafter to be also referred to as a green film) is imidated at a high temperature. In this case, the green film is heat-treated during feeding to perform drying and heat treatment. Such green film generally shrinks during drying. As an apparatus to feed a film while stretching tightly the film in the width direction by retaining the both ends in the width direction of the film with a number of pins and clips in such a drying•heat treatment of a green film, a tenter type feeding apparatus of a film, which is what is called a tenter, is known (see patent reference 1). In addition, use of a tenter type feeding apparatus for the production of a polyimide film is known in many cases (see patent reference 2).    patent reference 1: JP-B-39-029211    patent reference 2: JP-A-09-188763
Use of a film feeding apparatus for drying purposes to prevent development of wrinkles in the drying step after dyeing cloth has long been known well. In addition to drying of cloth, it is also used for drying an undried plastic film while feeding in a drying step during solvent film forming method. Using a film feeding apparatus, shrinkage of a film in its width direction due to the heat during the drying•heat treatment is suppressed, whereby development of wrinkles in the film after the drying•heat treatment due to the shrinkage can be prevented.
While the film shrinks not only in the width direction of the film but omnidirectionally, since a feeding tension acts on the feeding direction of the film, it has a suppressive effect on the shrinkage. In this way, using a tenter type feeding apparatus for feeding an undried film during a drying•heat treatment, the strength and flatness necessary for the film can be ensured.
Of the tenter type feeding apparatuses, a tenter type feeding apparatus that retains a film while stretching the film tightly in the width direction by allowing a number of pins to bite into the film along the both ends of the film is constituted by a pair of movement chains disposed in parallel, pin sheets lined up and supported thereby, and a number of pins set on the sheets. A feeding apparatus using pins is superior in the apparatus cost and downsizing of the apparatus because the structure is simple as compared to a feeding apparatus using clips and can be converted to a structure wherein the pathway of the feeding conveyor in a drying chamber is inverted. On the other hand, the number of the pins needs to be minimized where possible because small pieces of the film are produced as dust when the pins bite into the film. In addition, the apparatus has problems in that the pores made by the pins biting into the film surface are broken to form long pores in the width direction of the film when the shrinkage force of the film becomes high, and the like.
To improve them, a method including use of a film having high tear strength for a separate reinforcement to be superposed on the gripped portion of the film by the pins has also been proposed (see patent reference 3).
Moreover, a tenter type feeding apparatus for feeding a web and the like, wherein the density of the innermost side of the arrangement of pins on the both ends of the web is increased and the arrangement density of the outer side thereof is made smaller has been proposed (see patent reference 4). These proposed improvement plans still have problems in the production of a film in which pores made by the pins biting thereinto easily produce breakage to form long pores in the width direction of the film due to the shrinkage of the film.
That is, conventional tenter type feeding apparatuses are not sufficiently satisfactory for end users due to the problem of breakage of pores (made by the pins biting thereinto) to form long pores in the width direction of the film and easy development of quality failure such as development of wrinkles and the like, which cause production loss and low production efficiency.    patent reference 3: JP-A-11-254521    patent reference 4: JP-A-09-077315